Paste screener

ABSTRACT

The present invention relates to a screener for removing foreign substances contained in a paste, and a method for removing foreign substances contained in paste by using a screener.

FIELD OF THE INVENTION

The present invention relates to a screener for removing foreignsubstances contained in a paste, and a method for removing foreignsubstances contained in paste by using a screener.

TECHNICAL BACKGROUND OF THE INVENTION

When preparing a paste, a certain degree of foreign substances arecontained in the paste. Since the foreign substance might causedefective products, it is commonly required that it be removed from thepaste. For instance, a panel coated with a paste containing a foreignsubstance might turn out to be defective, which results in lower yieldof plasma display panel (PDP) production. In recent years, screen panelsare getting larger and larger, and the content of foreign substances inthe paste used for the panels is strictly regulated, in order to reducethe number of defective products.

For more efficient screening, of the paste, it is preferable to use afine screen mesh. When a fine screen mesh is used to remove the foreignsubstance, although the screening quality is improved, much more time isrequired. Therefore, there is a need for fast screening methods. Inparticular, the screening time tends to be prolonged if the paste ishighly viscous, and quick, effective screening methods are greatly indemand.

JP2002-239311A disclosed a screener provided with a feeding hole tosupply paste to be filtered, a squeegee which stirs the supplied pasteand presses it onto the screen mesh, a screen mesh which removes theimpurities in the paste, and a receiving tank which stores the pastepassed through the screen mesh. In JP2002-239311A, a flat squeegee isused (refer to FIG. 5).

However, the use of such a screener does not reduce the screening time.The present invention addresses the need for improvements in order toachieve reduced screening time.

SUMMARY OF THE INVENTION

The present invention relates to a screener comprising, in combination,a screen mesh which removes foreign substances in a paste, a squeegeewhich stirs the supplied paste and squeezes it onto the screen mesh, anda tank which stores the paste passing through the screen mesh,characterized in that the area of the underside of said squeegeeparallel with the screen mesh is 10 or above when the rotation area ofthe squeegee is 100 arbitrary units.

The present invention further relates to a method of removing foreignsubstances in a paste, comprising the steps of: providing a screenercomprising a screen mesh which removes the foreign substances in apaste; stirring the supplied paste with a squeegee, having a top sideand an underside, and squeezing, with the squeegee, the stirred pasteonto the screen mesh, wherein the paste that passes through the screenmesh is stored in a tank. The area of the underside of said squeegeeparallel with the screen mesh may be 10 or above when the rotation areaof the squeegee is 100 arbitrary units.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. Sectional view of a screener

FIG. 2. Diagram of the structure around the squeegee shown in FIG. 1.

FIG. 3. Diagram of a squeegee.

FIG. 4. Plain view of the squeegee shown in FIG. 3.

FIG. 5. Rotation area of the squeegee.

FIG. 6 shows the cross section of a part indicated as VI.

FIG. 7 shows the underside parallel to the screen mesh in the squeegeeindicated by S2.

FIG. 8 shows a bar shaped squeegee.

FIG. 9 shows the underside of the squeegee in FIG. 8.

FIG. 10 and FIG. 11 show increasing the width of the squeegee. FIG. 12shows a disc-like squeegee.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a means for shortening the time requiredfor screening out foreign substances in a paste. The screener in thepresent invention comprises a screen mesh to remove impurities in paste,a squeegee to stir the supplied paste and squeeze it onto the screenmesh, and a tank to store the paste passing through the screen mesh; thearea of the underside of the squeegee parallel to the screen mesh is 10or more when the rotation area of the squeegee is 100 units. The typicalscreen mesh size is in the range of 325 to 1,400. In one embodiment, themesh size is 800 to 1,400 mesh.

First, a sectional view of an embodiment of the screener of the presentinvention is shown in FIG. 1. The screener 10 has a screen mesh 100, asqueegee 102, and a tank 104. When the paste passes through the screenmesh 100, foreign substances such as aggregates, large particles, andforeign bodies are removed. The squeegee 102 stirs the paste suppliedfrom a feeder. Moreover, the paste is pressed against the screen mesh100 by the squeegee 102. Screening of the paste is completed by forcingthe paste towards the screen mesh with the squeegee 102. The paste whichpasses through the screen mesh 100 is stored in a tank 104. These arethe basic components of the screener of the present invention.

The screen mesh 100 serves to remove foreign substances. There is nospecial restriction on the shape and diameter of the screen mesh. Ahoneycomb-like mesh with many hexagonal openings is preferred from theviewpoint of screen mesh strength. The mesh diameter can be determinedaccording to the size of the foreign substance to be removed. Forinstance, to remove impurities ≧50 μm in diameter, a screen mesh ≧50 μmcan be used.

The squeegee 102 stirs the paste and squeezes it against the screenmesh. In a particular embodiment, the shape of the squeegee 102 isadjustable. The shape of the squeegee 102 is described in detail below.In a further embodiment, the squeegee 102 may be movable in the up anddown vertical directions. If the squeegee is movable, it can bepositioned at an appropriate height according to the type of the pasteand the shape of the squeegee.

In an embodiment, the squeegee 102 is not in contact with the screenmesh 100. When the screen mesh 100 is in contact with the squeegee, thatis, when they are in the on-contact state, the foreign substance mightbe squeezed into the screen mesh 100 by direct force from the squeegee.As a result, the foreign substances that are supposed to be removed bythe screen mesh 100 pass through the mesh 100 and might not be removedsufficiently. On the other hand, when the screen mesh 100 is off-contactwith the squeegee, that is, they are in the non-contact state, theaforementioned problems can be avoided. However, in the non-contact modethe time required for screening is longer, due to the insufficientpressure applied to the screen. The present invention can be used toscreen impurities in a comparatively short period of time even in theoff-contact mode, since the time required for screening is shortened.Namely, efficient screening is made possible by using the off-contactmode. Another benefit of the off-contact mode is the prevention of wearand tear on the screen mesh 100 and squeegee 102. If the screen mesh 100and squeegee 102 are in contact with each other, there is a risk thatpowder from the squeegee generated by the wear and tear of rotationmight get into the paste. Problems like this can be prevented with theoff-contact mode.

There is no special limitation on the gap between the screen mesh 100and the squeegee 102 in the case of non-contact operation. If the gap istoo wide, the squeezing force applied by the squeegee 102 to the screenmesh may be insufficient and the time required for screening might belong. Also, if the gap is too narrow it will be difficult to control soas to maintain an off-contact state. With these considerations in mind,it is preferred that the gap between the screen mesh 100 and thesqueegee 102 be between 15 μm to 35 μm. However, the scope of thepresent invention is not limited to this range.

The tank 104 stores the paste after passage through the screen mesh 100to remove the impurities. There is no special restriction on the sizeand configuration of the tank 104. The use of a tank 104 with a largecapacity allows processing a large amount of paste at one time. It isalso acceptable to install a tank drain to remove paste from the tankcontinuously, according to circumstances. Continuous long-time screeningis possible if such a tank drain is installed.

It is also acceptable to use a screener 10 with a paste feeder section106, paste container 108, motor 110, rotary shaft 112, support plate 114(refer to FIG. 2), and vacuum unit 116, if necessary.

The feeder section 106 supplies paste to the paste container 108. It isdesirable that the feeder section 106 be isolated from the externalatmosphere to prevent contamination by foreign substances from outside.There is no special restriction on the concrete configuration of thefeeder section 106. For instance, the configuration disclosed inJP2002-239311A can be adopted. JP2002-239311A is hereby incorporatedherein by reference in its entirety.

The paste container 108 temporarily holds the paste to be supplied tothe screener before passing through the screen mesh. There is no specialrestriction on the size of the paste container 108. It is sufficient tochoose its size in proportion to the amount of paste to be screened.

The motor 110 is the power source for rotating the squeegee. Althoughthe squeegee can be operated manually, if necessary. It is desirable touse a motor to rotate the squeegee, because the consistency of therotation and the screening uniformity are improved, and less manuallabor is required.

The rotary shaft 112 performs the task of transmitting power from themotor to the squeegee. In the mode shown in FIG. 1, the paste feedersection 106 surrounds the rotary shaft.

The tank 104 should be connected with the vacuum unit 116 to reduce thepressure in the tank. The speed with which the paste passes through thescreen mesh 100 can be improved by decreasing the pressure in the tankby using the vacuum unit 116. Moreover, air in high-viscosity paste canbe removed.

FIG. 2 is a diagram illustrating the structure around the squeegee. Asshown in the diagram, the screen mesh 100 is located under the pastecontainer 108. It is acceptable to place a support plate 114 under thescreen mesh 100 to prevent shifting and distortion while the screen meshis operating. Steady screening becomes possible with such a supportplate 114. Also, a retainer such as a clip 118 can be mounted to holdthe screen mesh 100 and support plate 114 in place.

A squeegee with the above properties is installed in the screener in thepresent invention. The type of squeegee used is such that if therotation area of the squeegee is 100, the area of the underside of saidsqueegee parallel to the screen mesh is 10 or more. By using a squeegeewith a large underside area parallel to the screen mesh, the timerequired for screening can be greatly shortened.

The rotation area of the squeegee refers to the area where the squeegeerotates and moves. For instance, the case of a squeegee with the shapeshown in FIG. 3 is illustrated. FIG. 3 is a model diagram of a squeegeein an embodiment. FIG. 4 is a plain view of the squeegee shown in FIG.3. The dotted line shows the tapered part of the underside of thesqueegee. When the squeegee shown in FIG. 4 rotates in the direction ofthe arrow, the rotation area (S1) is the area shown in FIG. 5. FIG. 6shows the cross-section of the part illustrated as “VI”.

On the other hand, the lower side parallel to the screen mesh means theside facing the screen mesh. When the screen mesh lies beneath thesqueegee, it means the area under the squeegee. To illustrate with FIG.6, the section indicated by A is equivalent to the underside parallel tothe screen mesh. In this case, the underside area parallel to the screenmesh in the squeegee is the area indicated by the section S2 in FIG. 7.

The time required for screening can be greatly shortened by increasingthe underside area parallel to the screen mesh when the rotation area ofthe squeegee is 100, calculated by the formula (S2/S1)×100.

It is preferable that (S2/S1)×100 be 10 or more, more preferably ≧20,≧30, ≧40, and ≧50, wherein the properties improve as the numbersincrease, and wherein ≧50 is the most preferred.

The squeegee may have various shapes known to one of skill in the art.Examples include the bar-shaped squeegee shown in FIG. 8 and theCrisscross squeegee shown in FIG. 3. For purposes of reference, theunderside area in the bar-shaped squeegee shown in FIG. 8, which isparallel to the screen mesh, is the area of the shaded section shown inFIG. 9. The ratio (S2/S1)×100 can be increased by increasing the widthof the squeegee as shown in FIG. 10 and FIG. 11.

In a particular embodiment, the value of (S2/S1)×100 is increased byusing the disc-like squeegee shown in FIG. 12. FIG. 12 is a plain viewof a disc-like shaped squeegee. The value of S2 can be increased byusing such a disc-like shaped squeegee.

When using this type of disc-like shaped squeegee, there may bedifficulties in feeding the paste under the squeegee, because of thelarge surface area of the squeegee. To solve this problem, an opening120 may be formed in the squeegee to supply the paste from underneath.The paste is supplied through the opening 120, which acceleratesefficient screening of the paste. Alternatively, a slit 112 can be madein the periphery of the disk and paste supplied through it.

The underside of the squeegee should be tapered at the edge in thedirection of rotation of the squeegee, regardless of the shape of thesqueegee. That is, it is preferable that the forward, rotating edges ofthe blades be slit downward as shown in FIG. 3, FIG. 6, FIG. 9, and FIG.10. With the formation of this type of taper, the paste stirred by thesqueegee is squeezed toward the underside of the squeegee, that is, ontoof the screen mesh. As a result, more efficient screening is achieved.

There are no particular limitations on the size of the squeegee.Nevertheless, for effective stirring and squeezing of the paste towardthe screen mesh, in a particular embodiment, the squeegee size may notbe much smaller than the width of the paste container. Therefore,although there is no special restriction, when the width of the pastecontainer is 100, the width of the squeegee should be ≧80, with ≧85better and most preferably ≧90.

The size of the squeegee can be determined based on the rotation area ofthe squeegee to the area of the screen mesh or the width of the pastecontainer. In this case, taking the width of the paste container or thearea of the screen mesh to be 100, although there is no specialrestriction the rotation area of the squeegee should be ≧70 with ≧75better and ≧80 best. There is no special restriction on the compositionof the squeegee. Rubber materials such as the polyurethane or resinssuch as polyacetal resin are suitable. In a particular embodiment,polyacetal resin squeegees are used. In an aspect of this embodiment,polyacetal resin squeegees are used for screening in the on-contactmode. The use of polyacetal resin squeegees for screening in theon-contact mode may prevent the formation of powders generated by thewear and tear of other squeegees.

In a further embodiment, polyurethane squeegees are used.

EXAMPLES

The relation between squeegee shape and screening time is evaluated inthe following experiments.

Paste Preparation

A silver paste was obtained by mixing the following components. Theviscosity of the paste was 40-50 Pa/s. Polymer resin 25 wt. % Silverpowder 60 wt. % Glass powder 5 wt. % Solvent 5 wt. % Additives 5 wt. %

Comparative Example 1

A 300 kg screener comprising a Crisscross-shaped squeegee, a motor, arotary shaft, a tank, a paste container, a screen mesh, and a supportplate as shown in FIG. 3 was prepared. The screen mesh and the supportplate were attached on the lower side of the paste container. Therotation area of the squeegee was 1194 cm². Also, the area of thesection parallel to the screen mesh under the squeegee was about 88 cm².That is, taking the rotation area of the squeegee to be 100 arbitraryunits, the underside area parallel to the screen mesh is 7.

The squeegee was lowered until it came in contact with the screen meshand then raised up 20 μm, so that the squeegee was in non-contact mode.

Paste was placed in the paste container and the screening was started.The rotation speed of the squeegee was 75 rpm. The paste to be screenedwas placed into the container gradually according to the screeningspeed. The time required for screening about 300 kg silver paste wasmeasured, and the processing rate (kg/hr) and the processing time(hr/Batch) were found. The processing time is the time required toprocess 300 kg of silver paste. The above procedure was repeated fivetimes, and the average value of the processing rate and the processingtime was calculated. The results are shown in Table 1.

Working Example 1

A squeegee wider than the one used in the Comparative Example 1 was used(refer to FIG. 9). The rotation area of the squeegee was 1194 cm² andthe area of the section parallel to the screen mesh under the squeegeewas about 240 cm². That is, taking the rotation area of the squeegee tobe 100, the underside area parallel to the screen mesh is 20. Except forthe different squeegee used, the screening conditions were identicalwith those in Comparative Example 1. The results are shown in Table 1.

Working Example 2

A squeegee wider than the one used in the Comparative Example 1 was used(refer to FIG. 9). The rotation area of the squeegee was 1194 cm² andthe area of the section parallel to the screen mesh under the squeegeewas about 633 cm². That is, taking the rotation area of the squeegee tobe 100, the underside area parallel to the screen mesh corresponds to53. Except that the squeegee was replaced, screening was conducted underthe same conditions as in Comparative Example 1. The results are shownin Table 1. TABLE 1 Comparative Working Working Example 1 Example 1Example 2 Squeegee cm² 1194 1194 1194 rotation area (S1) Area of the cm²88 240 633 squeegee parallel with the screen mesh (S2) Area ratio 7 2053 ((S2/S1) × 100) Gap Mm 20 20 20 Rotation rate Rpm/min 75 75 75Processing rate kg/hr 13.7 25.2 49.5 Operation time hr/Batch 21.9 11.96.1 Processing Modified/Initial 1 1.84 3.61 capacity

To facilitate comparison, the processing capacity is indicated in Table1 relative to the processing capacity of the Comparative example 1,taken as 1. As shown in Table 1, the time required for screening can begreatly shortened by increasing the area of the underside of thesqueegee. Specifically, taking the processing capacity in Comparativeexample 1 to be 1, (S2/S1)×100 in the working example 1 is 20 and theprocessing capacity will improve to 1.84. Additionally, for workingexample 2 with (S2/S1)×100 is 53, the processing capacity will improveto 3.61.

1. A screener for removing foreign substances in a paste comprising: ascreen mesh which removes the foreign substances in a paste; a squeegeewhich stirs the supplied paste and squeezes it onto the screen mesh; anda tank which stores the paste passing through the screen mesh, said tankcharacterized in that the area of the underside of said squeegeeparallel with the screen mesh is 10 or above when the rotation area ofthe squeegee is 100 arbitrary units.
 2. A screener as set forth in claim1, equipped with a vacuum unit for reducing the pressure in the tank. 3.A screener as set forth in claim 1, wherein the squeegee is off-contactwith the screen mesh.
 4. A screener as set forth in claim 1, wherein thearea of the underside of said squeegee is 20 or more when the rotationarea of the squeegee is 100 arbitrary units.
 5. A screener as set forthin claim 1, wherein the area of the underside of said squeegee is 50 ormore when the rotation area of the squeegee is 100 arbitrary units.
 6. Ascreener as set forth in claim 1, wherein the squeegee is bar-shaped. 7.A screener as set forth in claim 1, wherein the squeegee isCrisscross-shaped.
 8. A screener as set forth in claim 1, wherein thesqueegee is disc shaped.
 9. A screener as set forth in claim 8, whereinthe paste is supplied through an opening in the underside of thesqueegee.
 10. A screener as set forth in claim 1, wherein the undersideof the squeegee has a tapered edge in the direction of rotation.
 11. Ascreener as set forth in claim 1, wherein the squeegee is made ofpolyacetal resin.
 12. A method of removing foreign substances in apaste, comprising the steps of: (a) providing a screener comprising ascreen mesh which removes the foreign substances in a paste, (b)stirring the paste of (a) with a squeegee, said squeegee having a topside and an underside, and (c) squeezing, with the squeegee, the stirredpaste onto the screen mesh, wherein the paste that passes through thescreen mesh is stored in a tank and wherein the area of the underside ofsaid squeegee parallel with the screen mesh is 10 or above when therotation area of the squeegee is 100 arbitrary units.
 13. The method ofclaim 12, wherein the screener is equipped with a vacuum unit forreducing the pressure in the tank.
 14. The method of claim 12, whereinthe squeegee is off-contact with the screen mesh.
 15. The method ofclaim 12, wherein the area of the underside of said squeegee is 20 ormore when the rotation area of the squeegee is 100 arbitrary units. 16.The method of claim 12, wherein the area of the underside of saidsqueegee is 50 or more when the rotation area of the squeegee is 100arbitrary units.
 17. The method of claim 12, wherein the squeegee isbar-shaped.
 18. The method of claim 12, wherein the squeegee isCrisscross-shaped.
 19. The method of claim 12, wherein the squeegee isdisc shaped.
 20. The method of claim 12, wherein the paste is suppliedthrough an opening in the underside of the squeegee.
 21. The method ofclaim 12, wherein the underside of the squeegee has a tapered edge inthe direction of rotation.
 22. The method of claim 12, wherein thesqueegee is made of polyacetal resin.
 23. A paste screened by the methodof claim
 12. 24. A paste passed through the screener of claim 1.